1. Field of the Invention
The present invention relates to a method of reducing the uncertainty when measuring the hydraulic conductivity and the specific storativity of a rock sample.
2. Description of the Related Art
In order to prevent the contamination of various landfills from being spread when underground water resources or petroleum resources are developed, in order to investigate an underground bedrock environment, or in order to perform other various civil engineering works, site investigations are required. Among them, representatively, the measurement of the permeability or storativity of bedrock is required.
In this case, the permeability of the bedrock refers to the easy liquidity of a fluid flowing through fluid passages communicating with each other through gaps in the bedrock. Since the flow of the fluid through the fluid passage receives the resistance by attraction force or pressure, permeability may be represented differently depending on bedrocks or stratums.
In addition, the storativity of bedrock refers to the characteristics of the bedrock related to the storage capacity of a fluid when injecting the fluid through fluid passages communicating with each other through the gaps in the bedrock. If the potential energy or the kinetic energy is not changed, the fluid does not flow, but is maintained in a stationary state.
In general, when a fluid (representatively, water) inflows into or outflows from an underground rock formation or a stratum (hereinafter, collectively, rock formation), the diffusion of the pressure of the fluid may be affected by the permeability or the storativity of rocks mainly constituting a bedrock or a related stratum.
In this case, the permeability or the storativity of the bedrock is an essentially required parameter to estimate the inflow/outflow place of the fluid or estimate the variation (diffusion) of the fluid according to the elapse of time.
Since the permeability or the storativity of the rock formation is a characteristic generated in an underground natural state, the real permeability or storativity of the rock formation cannot be actually detected. Accordingly, generally, required data are acquired through a hydraulic test in a laboratory.
The permeability or the storativity may be measured through various schemes. When rocks constituting the rock formation are densely formed and have impermeability, a pulse-decay (transient) scheme is used to apply pressure pulses to a rock sample for the measurement in order to reduce the measurement time.
As representative values having the numerical characteristics of a rock sample constituting a target rock formation through the hydraulic test in the above laboratory, hydraulic conductivity and the storage coefficient can be calculated.
In this case, the hydraulic conductivity is defined as the passing speed of water, that is, a value obtained by dividing a passing distance of water by time. As the value of the hydraulic conductivity is reduced, the flow of the fluid (water) is more difficult in the underground rock formation.
In addition, the storage coefficient is defined as the quantity of water per area inflowing into or outflowing from an aquifer due to the variation of the unit hydraulic head.
Meanwhile, the specific storativity (specific storage) is defined as the quantity (volume) of water inflowing into or outflowing from the aquifer due to the increase or the drop of the unit hydraulic head per unit volume in the aquifer. If the thickness of the aquifer is multiplied by the specific storativity, the storage coefficient can be found.
According to the pulse-decay scheme of the related art, only the hydraulic conductivity is measured under the assumption that the specific storativity is 0, or the hydraulic conductivity and the specific storativity are calculated by inversely calculating the pressure curve.
However, the first problem in the scheme of obtaining the hydraulic conductivity and the specific storativity according to the related art is that the degree of the uncertainty of each parameter is not provided.
In other words, since an experimental error essentially exists in the pressure measured in the hydraulic test of a laboratory, both of the hydraulic conductivity and the specific storativity inevitably have a slight degree of uncertainty. However, according to the related art, since data related to the uncertainty are not provided, the reliability of the experiment cannot be recognized.
The second problem is that the uncertainty of a parameter resulting from the experimental error is varied depending on an experimental system (the sizes of the upper and downstream reservoirs; see reference numerals 10 and 20 of FIG. 1 in relation to the upper and downstream reservoirs). In this case, the condition used to find the parameter cannot be recognized.
The third problem is that the uncertainties of parameters are affected by each other according to the conditions of the hydraulic test. If the uncertainty of any one parameter is unknown, the uncertainty of another parameter is not recognized.
The scheme of finding the hydraulic conductivity and the specific storativity according to the related art does not provide solutions to the above problems.
Meanwhile, there are following non-patent documents (papers) as documents of the related art.
(1) Brace, W. F., J. B. Walsh, and W. T. Frangos, Permeability of granite under high pressure, J. Geophys. Res. 1968 73 2225-2236.
(2) Hsieh, P. A., J. V. Tracy, C. E. Neuzil, J. D. Bredehoeft, and S. E. Silliman, A transient laboratory method for determining the hydraulic properties of tight bedrocks-I. Theory, Int. J. Bedrock Mech. Min. Sci. & Geomech. Abstr. 1981 18 245-252.
(3) Wang, H. F., and D. J. Hart, Experimental error for permeability and specific storage from pulse decay measurements, Int. J. Mech. Min. Sci. Geomech. Abstr. 1993 30, 1173-1176.
(4) Zhang, M., M. Takahashi, R. H. Morin, and T. Esaki, Evaluation and application of the transient-pulse technique for determining the hydraulic properties of low-permeability bedrocks-Part 2: Experimental application, Geotechnical Testing Journal 2000b; 23, 091-099.